This application claims the priority of German application 198 52 294.0, filed Nov. 12, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an exhaust system of a multi-cylinder internal-combustion engine having at least one system part in which the internal-combustion engine exhaust gases or portions thereof are first guided through at least two partial pipe trains apportioned to cylinder groups, in which partial pipe trains, one starting catalyst respectively is inserted and which combine to form a joint main pipe in which a main catalyst is inserted, at least one lambda probe being arranged in front of and one lambda probe being arranged behind the catalysts.
In the course of the tightening of the emission control laws, an optimal pollutant reduction of internal-combustion engines is becoming increasingly important. The aftertreatment of the exhaust gases in a catalyst is known. For an optimal operating method of a catalyst, a favorable exhaust gas composition must be ensured which takes place by a lambda control known per se. In the simplest case, a lambda probe is arranged in front of a catalyst and emits a signal to a control which, on the basis of this signal and the power demand, controls the fuel charge into the cylinders of the internal-combustion engine.
According to the Bosch Manual, 22nd Edition, VDI Publishers, Dusseldorf, starting on Page 490, a lambda control can take place according to the two-point method, in the case of which an adjusting quantity changes its adjusting direction at each voltage jump which indicates a rich/lean or lean/rich change. Despite such a two-point control, aging and environmental influences (contaminations) act as a disturbing influence on the precision of measurements. For this purpose, it is known to arrange another lambda probe behind the catalyst which is subjected to the above-mentioned influences to a significantly lower extent. In the case of the principle of the two-probe control, the controlled rich or lean displacement is additively changed by a correction control loop.
In the case of engines with a low number of cylinders (up to four cylinders), a single-flow exhaust gas system, that is, an exhaust gas system with one pipe train, can be used. In the case of engines with a higher number of cylinders, the use of a two-flow exhaust gas system is more favorable in the sense of a better full-load action. However, such a completely two-flow exhaust gas system is expensive and has a poor starting behavior with respect to the pollutant reduction. As an alternative, an exhaust gas system was found to be advantageous which is constructed in a two-flow manner only in its forward portion; that is, the exhaust gases are first guided through at least two partial pipe trains divided into cylinder groups, which partial pipe trains are then combined to form a common main pipe. Such an exhaust gas system is also involved in this case.
The exhaust gas catalysts will only reach their optimal effect if they are in a certain temperature range (for example, from 400 to 800.degree. C.). The heating of the catalyst particularly presents problems in the starting phase. In order to accelerate the heating, among other provisions, smaller precatalysts are used which are arranged in the proximity of the cylinders and can be brought particularly rapidly to their operating temperature. When different partial pipe trains or a multi-flow exhaust gas system are used, a pertaining precatalyst or starting catalyst is used for each partial pipe train. Reference is made in this context to German Patent Document DE 195 24 980 A1.
It is an object of the present invention to provide an exhaust gas system of the initially mentioned type which permits a precise adjusting of the air-fuel mixture.
This object is achieved according to preferred embodiments of the invention by providing an exhaust system of the above-noted type, wherein a lambda probe is arranged in front of each starting catalyst and, at least in one partial pipe train, an additional lambda probe is arranged behind the starting catalyst.
When using several precatalysts as well as well as lambda probes, which are in each case arranged in front of them, and another trimming or adjusting lambda probe behind a main catalyst, it presents a problem that this trimming or adjusting lambda probe detects the exhaust gases from all partial trains which are brought together in the main train. It is therefore possible that the exhaust gases may mix in such a manner that occurring lambda differences are compensated. In any case, a deviation can no longer be determined directly and can also no longer be assigned to a certain lambda probe in front of a starting catalyst or precatalyst.
For avoiding this disadvantage, on the one hand, an is additional lambda probe is provided in front of each starting catalyst and, on the other hand, at least in one partial pipe train, another lambda probe is arranged behind the starting catalyst. Depending on the exhaust gas system architecture, one or several of such system parts can be used in parallel or be interconnected.
The signals of the above-mentioned lambda probes are fed to a control which, on the basis of this information, can determine exactly those partial pipe trains or precatalysts through which a fuel-air ratio is guided which is not optimal. This permits measures for returning these unintended deviations in the individual cylinders to zero.
According to an advantageous feature of certain preferred embodiments of the invention, not all partial pipe trains have to be monitored by additional lambda probes connected behind the starting catalysts.
According to certain preferred embodiments of the invention, in order to be able to monitor all precatalysts and lambda probes connected in front of the latter, however, in the case of n partial pipe trains, n-1 additional lambda probes should be provided behind the starting catalysts.
The lambda probes in front of the starting catalysts are preferably constructed as linear lambda probes or broad band probes. The lambda probes behind the starting catalysts may be constructed as jump probes.
On the whole, by means of the lambda probes arranged behind the starting catalysts, the respective lambda probes situated in front can be trimmed or adjusted. By means of the lambda probe arranged behind the main catalyst, an overall monitoring or a monitoring of a last remaining partial train can also be achieved without any additional lambda probe. On the whole, the overall system has adjustability with respect to .lambda.=1 or .lambda.&gt;1 concepts.
As a rule, the monitoring of the precatalyst function takes place by a temperature comparison between the temperatures in front of and behind the catalyst. For this purpose, two temperature sensors are required as a rule for each partial pipe train. In the case of the present invention, as an alternative, a precatalyst or starting catalyst can also be monitored by the comparison of the lambda signals in front of and behind the catalyst. In the case of a partial pipe train where no additional lambda probe is provided, a temperature probe or a temperature sensor can be arranged behind the respective precatalyst.
On the whole, the exhaust system according to the invention results in a good lambda adjustability together with a good full load behavior. In addition, a low-cost and light exhaust system can be implemented which is easy to package and has a fast starting and heat-through behavior. Also, for lean concepts (.lambda.&gt;1), there is the advantage of a lower consumption in comparison to a continuous two-flow system Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.